1. Field of the Invention
The present invention relates to a cast explosive primer adapted to be initiated instantaneously by low-energy detonating cord (LEDC), and to an assembly of an LEDC downline with the primer for initiating a cap-insensitive explosive in a borehole.
2. Description of the Prior Art
Low-energy detonating cord (LEDC), which may have an explosive core loading of only about 0.2 to 2 grams per linear meter of cord length, is widely used in non-electric explosive initiation systems in cases in which the noise and high brisance of heavier cords must be avoided. When used heretofore in conjunction with high-energy primers to initiate cap-insensitive explosives in boreholes, LEDC downlines have been used with primer or booster units containing a percussion-actuated detonator, e.g., in the delay booster assembly described in U.S. Pat. No. 3,709,149, issued Jan. 9, 1973, to H. E. Driscoll. U.S. Pat. No. 4,718,345 describes a primer assembly which includes a percussion-actuated detonator seated in a cavity in a high-energy primer, and an explosive coupler for explosively coupling the detonator to LEDC which is to be threaded through a perforation in the primer. This assembly, when incorporating a delay detonator, can be used in an in-hole delay system for the delayed initiation of deck-loaded explosive charges with high-energy delay primers strung on a single LEDC downline.
For the instantaneous initiation of a primer by an LEDC downline, co-pending, co-assigned U.S. patent application Ser. No. 035,004, filed Apr. 6, 1987, describes a primer unit requiring no detonator while relying only on an external arming element to achieve reliable initiation of the primer explosive by the low-energy cord. The arming element, i.e., an explosive coupler containing a charge of granular detonating explosive, is attached to a preferably recessed end surface of the primer and is manually disengageable therefrom. The explosive charge is linearly arrayed perpendicular to the cord and, at least at the end adjacent to the cord, is sufficiently shock-sensitive as to pick up the detonation from the low-energy cord, while boosting the energy level of the detonation to initiate the explosive compound in the adjacent primer.